Gyroscopes are spinning bodies, such as wheels or discs, that generate angular momentum as they spin about an axle. The principles of conservation of angular momentum make gyroscopes resistant to torques that are applied to their axles. Because of their tendency to reliably remain in a given orientation when spinning, gyroscopes have been used in various navigation systems.
One common application has been to use gyroscopes to control the movement of spacecraft operating in a weightless environment, such as that encountered while in orbit around the earth. The attitude of a spacecraft is controlled and or maneuvered gyroscopically by following the law of conservation of momentum. CMGs exchange their momentum with the momentum of the spacecraft. They do this by torquing the gimbal axis of the CMG to change the direction of the momentum vector of the CMG which then changes the momentum of the spacecraft. This changes the velocity of the spacecraft. Because gyroscopes are resistant to changes in the angle of their axles, a torque applied to the axle by a spacecraft operating in zero gravity will result in movement of the spacecraft around the gyroscope. In this manner, the attitude and orientation of the spacecraft can be controlled. Gyroscopes that are used for this purpose are known as control moment gyroscopes. The faster that a control moment gyroscope spins and the more mass that a control moment gyroscope has, the more resistant a control moment gyroscope will be to a torque applied to its axle.
The control moment gyroscope typically includes a rotor disposed within a housing. The rotor typically includes a shaft that serves as the rotor's axle, a rim that has a circular or ring-like configuration and which comprises the majority of the rotor's mass, and a web that connects the rim to the shaft. The shaft is configured to mount the rotor to the housing in a manner that permits the rotor to spin with respect to the housing.
The web may be is connected to the shaft and to the rim by one or more weld joints. As the rotor spins, the weld joints are placed in tension. The maximum speed at which the rotor can spin is therefore limited by the amount of tension stress that the weld joints can tolerate. Additionally, when torque is applied to the shaft to move the spacecraft, the weld joints alternately cycle through tension and compression due to the spinning of the rotor. Such cycling through alternate states of tension and compression can fatigue the weld joints and reduce their tolerance to tension which can, in turn, reduce the maximum rotational speed of the rotor.